Wake the GM: process approval comments and launch a focused session.

Gib's workflow: comment "approved" on standup doc → hit this endpoint → agent processes approvals and executes approved work → Gib gets email with results.

An optional JSON body {"prompt": "..."} hands the session a custom prompt (via the GM_SESSION_PROMPT env var gm_session already reads). The bare wake stays unauthenticated — it only triggers the agent's own standing flow — but a custom prompt is arbitrary instruction injection into a credentialed session, so it requires the admin key.

Launches the session as an ECS task (same pattern as pipeline launcher). Returns immediately with task info — the session runs asynchronously.